1. Field of the Invention
The present invention is an adjusting device of the light emitting diode (LED) backlight module, and more particularly is to provide the most suitable pulse width modulation (PWM) in each of the channels of the LED backlight module before driving the LED backlight module.
2. Description of the Prior Art
The backlight module of the large scale Liquid Crystal Display (LCD) TV is using Cold Cathode Fluorescent Lamp (CCFL) or Light Emitting Diode (LED) to be the light source. Because the CCFL light tube implements the mercury (HG) to be the illuminant light source, the mercury will cause the environmental protection problem during manufacturing and recycling. In addition, the CCFL tube must be isolated from air to increase the life time. The LED technology is well developed and the illuminant efficiency is better than the CCFL tube and the LED technology is flexible and easy in color and illuminant control. Therefore, the direct backlight module made by LED is going to substitute the CCFL tube to be the backlight module in LCD.
Please referring to FIG. 1A, it is a view illustrating a direct backlight module made by conventional LED. As shown in FIG. 1A, the direct backlight module 400 is made by several illuminant channels (401-40n, n is an integral) and each of the illuminant channels 401 includes many LEDs 500. One of the significant drawbacks in the LED direct backlight module is the illuminant of the individual LEDs is not all the same, especially when red light, green light and blue light LEDs are together to generate white light. The color temperature of the white light is difficult to control. Moreover, the illuminant of the different color light LED includes different temperature reaction. When LED has been worked for a period time, the temperature of LED is increased as time goes on and the illuminant difference in each of the LED is increased. For example, when the room temperature is increased over 80° C., the attenuation of the red light LED is more than the blue light LED, and the attenuation of the blue light LED is more than the green light LED. Therefore, the direct backlight module made by several LEDs is easily affected by the different color LED so as to vary the color temperature and the even illumination.
Moreover, in prior art, the analog driving circuit of the LED is configured to drive the direct backlight module transmits the control signal generated by the triangle wave generator and the amplifier to the DC-DC converter 600 (such as buck type or boost type DC-DC converter) so as to control turning the LED on or off. When the DC-DC converter 600 is configured to drive the LED array, the lumen in each of the LED is varied in accordance with the difference of the forward bias. Therefore, it is difficult to control the color temperature and the illuminant of the LED array.
In addition, in order to determine the voltage variation in each of the LED, some technologies utilizes the look-up table. However, those methods are required lots of memory, and those technologies are hard to be embedded in one chip.
Now, please referring to FIG. 1B, it is a block diagram illustrating the DC-DC converter of the LED backlight module in prior art. As shown in FIG. 1B. The voltage signal transmitted to the output voltage illuminant device 400 is that the pulse width modulation input and the output voltage are modulated by the current control circuit 53; the pulse width modulation (PWM) signal in each of the light channel is controlled to be transmitted to the select circuit 52 so as to select a pulse high signal at minimum conduction and at final, one voltage output (Vout) is transmitted to the illuminant device 400. Therefore, when the PWM signal is turned on at duty cycle, each of the light channels absorbs the current at the output voltage V (Vout). When the PWM signal is turned off at duty cycle, each of the light channels is closed. In prior art, a clamp circuit (not shown) is used to keep providing a stable voltage when the duty cycle is off. According to the description above, the PWM signal generated by the DC-DC converter and used to control each of the light channels uses the same frequency, the same phase and the same duty cycle to drive the illuminant device 400, as shown in FIG. 1C (the equivalent circuit of the illuminant device in FIG. 1B). Obviously, the equivalent circuit in FIG. 1C is not able to change the duty cycle and the phase of the PWM signal. It is not necessary to drive the illuminant device 400 by the PWM signal with the same duty cycle and the same phase. The better method is to provide proper PWM signal in accordance with the actual variation at each of the light channels.
Moreover, in order to overcome the problem that different phase in FIG. 1C cannot be solved; another conventional technique is to include a VFB resistor and a holding circuit in FIG. 1D so as to change the phase of the PWM signal. Therefore, the illuminant device 400 includes the PWM signal with different phase to drive the illuminant device. However, the difficulty of the circuit design is increased and a feedback pin is added in the chip, so the manufacture cost is increased.